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One way to customize Bazaar’s behaviour is with hooks. Hooks allow you to
perform actions before or after certain Bazaar operations. The operations
include commit, push, pull, and uncommit.
For a complete list of hooks and their parameters, see Hooks in the User Reference.

Most hooks are run on the client, but a few are run on the server. (Also
see the push-and-update plugin that handles one special case of
server-side operations.)

To use this example, create a file named push_hook.py, and stick it in
plugins subdirectory of your configuration directory. (If you have never
installed any plugins, you may need to create the plugins directory).

That’s it! The next time you push, it should show “The new revno is...”.
Of course, hooks can be much more elaborate than this, because you have the
full power of Python at your disposal. Now that you know how to use hooks,
what you do with them is up to you.

The plugin code does two things. First, it defines a function that will be
run after push completes. (It could instead use an instance method or
a callable object.) All push hooks take a single argument, the
push_result.

Second, the plugin installs the hook. The first argument 'post_push'
identifies where to install the hook. The second argument is the hook
itself. The third argument is a name 'Mypost_pushhook', which can be
used in progress messages and error messages.

To reduce the start-up time of Bazaar it is also possible to “lazily” install hooks,
using the bzrlib.hooks.install_lazy_named_hook function. This removes the need
to load the module that contains the hook point just to install the hook. Here’s lazy
version of the example above:

Here’s a complete plugin that demonstrates the Merger.merge_file_content
hook. It installs a hook that forces any merge of a file named *.xml
to be a conflict, even if Bazaar thinks it can merge it cleanly.

merge_xml.py:

"""Custom 'merge' logic for *.xml files.Always conflicts if both branches have changed the file."""frombzrlib.mergeimportPerFileMerger,Mergerdefmerge_xml_files_hook(merger):"""Hook to merge *.xml files"""returnAlwaysConflictXMLMerger(merger)classAlwaysConflictXMLMerger(PerFileMerger):deffile_matches(self,params):filename=self.get_filename(params,self.merger.this_tree)returnfilename.endswith('.xml')defmerge_matching(self,params):return'conflicted',params.this_linesMerger.hooks.install_named_hook('merge_file_content',merge_xml_files_hook,'*.xml file merge')

merge_file_content hooks are executed for each file to be merged. For
a more a complex example look at the news_merge plugin that’s bundled with
Bazaar in the bzrlib/plugins directory.